1. Field of the Invention
The present invention relates to stress control structure for optical fibers in regions of high electrical stress down to a lower electrical stress, or zero electrical stress or ground. In particular, it involves selections and arrangements of dielectric layers and other layers associated with optical fibers originating elsewhere or originating in or traversing regions of high electrical field strength and the control of the stress to which the optical fibers are exposed.
2. Description of the Art
The advantages of optical fibers over other media for the conveyance of digital and electrical signals are well known. However, optical fibers placed in regions of high electrical field strength are susceptible to damage and failure from localized dielectric breakdown of the electrical insulation system in the vicinity of the optical fibers. This condition is intensified when a discontinuity exists in the shielding of a high voltage cable, sensing device or other electrical equipment.
The following and all other referenced patents and applications are incorporated herein by reference in their entirety. Definitions herein prevail over definitions in such references.
Some of the prior art is very useful in pointing out solutions to various problem areas. U.S. Pat. No. 6,215,940, for High Voltage Insulator for Optical Fibers, issued Apr. 10, 2001, which reference is incorporated herein, describes the various embodiments of flexible insulator sleeves, characteristics of the insulative support rod, (corresponding to insulative central structural element herein), preserving integrity of the optical fibers, cushioning between the optical fibers and the support rod, problem of voids, (air gaps), moisture, and other aspects. This reference also points out that the length of the rod, (insulative central structural element herein), depends primarily upon the level of the (voltage) system on which it is used.
Also, U.S. Pat. No. 6,015,625, for Stress Control for Termination of a High Voltage Cable, issued Jan. 18, 2000, is incorporated herein by reference, providing test and design information as to stress control layers, including permittivity requirements, with improved corona discharge extinction, withstanding of power frequency voltage and impulse voltages, and in meeting the requirements of IEEE Standard Test procedures, STD 48-1990. That patent also provides helpful information as to exterior elastomeric coverings, (sleeves), and other types of exterior coverings, elimination of air pockets, as well as superiority of stress control material comprised of epihalohydrin rather than EPDM.
Elements:                1. Insulative Central Structural Element: The insulative central structural element is the innermost portion of the particular structure. Although other shapes may be used, it may be constructed of a longitudinal, central, circular, or somewhat circular, column, forming the body of the elongated portion of the structure. Whereas circular or near circular is the preferred shape, elliptical, triangular, square, hexagonal, octagonal and other multi-sided symmetric and asymmetric shapes might conceivably be used.                    The insulative central structural element is formed from a material with good insulative properties at high voltages. In various embodiments of the invention, the structure may be comprised of ceramic, glass, porcelain, plastic, polycarbonate, phenol, or other materials which are suitably insulative and provide the strength required to support the physical structure. Although it is not necessary, the insulative central structural element may be made of high dielectric material.            In one embodiment, such structure may be a high voltage cable with all it accompanying layers.            The thickness of the insulative central structural element is determined by the dielectric strength of the insulator material, the applied high-voltage and the needed safety factor as determined by the application. Typical thickness for the insulative central structural element is between 0.5 and 1.25 inches for an applied high-voltage of 35 kV.            Voltages in high voltage distribution systems range from 4 kV to 75 kV with common voltages of, say, only as examples, 15 kV, 25 kV, 35 kV, 46 kV and 69 kV. The dielectric capability and size of the insulative central structural element would, of course, take into account differences in intended use.                        2. Stress Control Layer: The stress control layer consists of a high permittivity material that is tightly-fitted around the insulative central structural element in such a way so as to create a void-free or substantially void-free interface. This high permittivity material manages the high-electrical stresses by creating a more evenly distributed e-field over the surface of the structure, significantly reducing the potential for insulation breakdowns that could result in partial discharge, withstand or Basic Insulation Level (BIL) failures. (Partial discharge occurs when a localized dielectric breakdown of the electrical insulation system under high voltage stress but not of sufficient magnitude to bridge the space between two conductors.)                    The thickness of the stress control layer is typically 50 to 200 mils and in various embodiments of the invention, the material may be comprised of Ethylene-Propylene (EP), Ethylene-Propylene-Diene-Monomer (EPDM), rubber or silicon or other suitable materials having properties, with various additives, fillers and conductive particles so as to provide the needed permittivity.                        3. Dielectric Layer: The dielectric layer consists of an elastomeric material with high dielectric strength that, in conjunction with a second layer of elastomeric material, form two layers that encapsulate the optical fibers, minimizing voids in the vicinity of the fibers. In one preferred embodiment, two dielectric layers are composed of the same material with self-fusing properties to minimize voids and with sufficient pliability to conform to the fiber radius; however the self-fusing property is not strictly required, but it is an advantageous embodiment.                    In various embodiments of the invention, the dielectric layer may be comprised of rubber, silicon, or other elastic materials with various additives and fillers to provide the needed dielectric material properties.                        4. Optical Fiber: The optical fiber is a cylindrical dielectric waveguide, typically made of low-loss glass or plastic comprised of a central core which guides the optical signal surrounded by a cladding material of slightly lower refractive index. Light impinging on the core-cladding interface at an angle greater than the critical angle undergoes total internal reflection and is guided through the core. Fiber optical cable is familiar to those skilled in the art. The optical fiber, in this application, is used for the transmission of analog or digital data through a region of high electrical stress. “Light” as used herein includes, of course, much more of the electromagnetic spectrum than visible light. Both infrared and ultraviolet frequencies might be used. Infrared and near infrared are most common. Such wider meaning is commonly known in the art.        5. Dielectric Layer: The dielectric layer consists of an elastomeric material with high dielectric strength that, in conjunction with a first layer of elastomeric material, form two layers that encapsulate the optical fibers, minimizing voids in the vicinity of the fibers.        6. External Insulator: An external layer of insulative material is placed around the structure. The external insulator is very tightly-fitting. The external insulator may include “skirts” or “sheds” or “wings” to decrease the susceptibility to external arcing or tracking; known to those skilled in the art, or, as described, for example in U.S. Pat. No. 6,215,940. The external insulator must prevent electrical tracking and must be strong, tight-fitting, hydrophobic and be an effective “weathershed”, providing protection against weather and harsh environmental conditions. It may be composed of highly insulative rubbers, silicone, various polymers and other suitable materials.                    Depending of the application, the external insulator may not be required since this is primarily necessitated by exposure to weathering and harsh environmental conditions. In various embodiments of the invention, this layer can be eliminated or replaced with an alternate external layer suitable for the physical conditions to which the structure will be exposed.            In the preferred embodiment, the construction elements 1-6, as referenced above, describe the materials and methods used to construct the stress control insulating device of the invention. It is conceivable that either one or both of the dielectric layers (elements 3 and 5 above) could be omitted. In alternate embodiments of the invention, the various construction elements could be combined, performing the same physical and electrical functions, albeit with a reduced number of physical elements, but accomplishing the same functions.                        
A description of a current and voltage sensor, such as might be used in combination with the inventions herein, is found in patent application Ser. No. 135534886, filed Jul. 19, 2012, entitled, “OPTICAL SENSOR ASSEMBLY FOR INSTALLATION ON A CURRENT CARRYING CABLE. (Applicant will furnish updated information when received.) Such application is incorporated herein for reference purposes.
The various items used in the electrical constructions herein are commercially available. Most can be ordered by size, length, width, dielectric constant, conductivity and other characteristics. The needs for cable elements can be met over a wide range of voltages. Many suitable products are available from several suppliers. Of course, a wide range of optical fibers are also readily available.